ELECTRIC
ROCKET ENGINE BASICS:
What
the different types of electric rocket engines look like and how
they work.

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Chemical
rocket engines, like those on the space shuttle, work by burning two
gases to create heat, which causes the gases to expand and exit the
engine through a nozzle. In so doing they create the thrust that
lifts the shuttle into orbit. Smaller chemical engines are used to
change orbits or to keep satellites in a particular orbit.

For getting to
very distant parts of the solar system chemical engines have the
drawback in that it takes an enormous amount of fuel to deliver the
payload. Consider the Saturn V rocket that put men on the moon:
5,000,000 pounds of it's total take off weight of 6,000,000 pounds
was fuel.

Electric
rocket engines use less fuel than chemical engines and therefore hold
the potential for accomplishing missions that are impossible for
chemical systems. To understand how, we have to understand a number
called specific impulse.

Take one pound
of the hydrogen/oxygen propellants used by the shuttle's main engines
and burn it in one second and you'd generate 370 pounds of thrust.
That "370" is a measure for the combined efficiency of the
engines and the propellants burned in them. If we could discover a
different chemical combination that produced twice as much thrust for
the same amount of fuel, it would have a specific impulse of 740. The
units of specific impulse are "seconds." The advantage of
having a higher specific impulse engine is that you need considerably
less fuel to accomplish the mission. Take the Saturn V rocket, if it
had a specific impulse of 700 seconds and could still produce the
same level of thrust it would only need 2,500,000 pounds of
propellant. Take that much weight off it and the structure could be
made lighter, which would mean that it would need even less fuel, and
so on. Reducing the weight of the fuel by increasing the specific
impulse is one of the most powerful ways of reducing the overall
weight of a spaceship.

The problem is
that all the energy for chemical engines comes from the energy stored
in the propellants. There is a limit to this and we've already pretty
much reached it with the shuttle engines. The way around this problem
is simple: we cheat.

Instead of
relying only on the energy stored in the propellants, we can add
energy using electricity. In the simplest concept an electric heater
is used to increase the temperature of the propellant above what it
could get through combustion. The higher the temperature the greater
the expansion and the more thrust per pound of propellant is
obtained. Because the energy no longer comes from the propellants, we
also now have a much wider range of propellant options since they no
longer have to be able to combust.

Sounds great
but there are three problems. First, you need a power supply to
provide the electricity and this adds weight to the rocket ship. (If
the specific impulse is high enough the propellant needed is reduced
so much that even with the power supply weight added in the total
ship weight is reduced.) Second, because space power systems only put
out small amounts of power compared to chemical engines, the amount
of thrust electric rocket engines produce is very small: on the order
of pounds or even fractions of a pound instead of the tens of
thousands to millions of pounds of thrust chemical engines produce.
(Electric engines make up for this by running for months or years
instead of minutes like chemical engines. In so doing the total
impulse (amount of thrust multiplied by the time the engine burns)
can actually be greater.) Third, electric rocket engines only work in
the vacuum of space because atmospheric pressures hinder the physical
processes by which they create thrust.

In spite of
these problems, millions of dollars are being invested every year to
develop electric rocket engines because in the long run they are
destined to play a major roll in humankind's conquest of space. In
fact, dozens of small electric thruster are already in space
performing many low-thrust missions much better than their chemical counterparts.

As the
specific impulse provided by an engine increases, the mass of
propellant needed decreases for a specific mission. Going hand in
hand with this is the fact that as specific impulse increases so does
the amount of electrical power needed to run the engines (at the same
thrust) so the mass of the power supply increases, offsetting the
savings realized from reducing the propellant. Plotting the combined
propellant and power supply mass results in a graph that looks like a
"U," where the bottom-most part of the "U" is the
optimal point where total spacecraft weight is at a minimum.
Spacecraft designers use this graph to determine the optimum specific
impulse and power for the mission in question. While the number
varies with thruster efficiency, power supply density and so on, as a
general rule of thumb you want specific impulses on the order of
300-1,000 sec for orbit maintenance missions, 1,000-2,000 sec for
orbit transfers, and 2,000-6,000 sec for interplanetary missions.

So, what do
these electric rocket engines look like and how do they work? The
following simple diagrams and explanations will help introduce you to
this interesting field:

Electrothermal
Rocket Engines:

This
class of electric rocket engine works by heating a propellant.

Resistojets:

A
resistojet simply uses electricity passing through a resistive
conductor, something like the wires in your toaster, to heat a gas as
it passes over the conductor. As the conductor heats up the gas is
heated, expands, exits through a nozzle and creates thrust.

diagram of a resistojet

In
real resistojets the conductor is a coiled tube through which the
propellant flows. This is done to get maximum heat transfer from the
conductor to the propellant.

Almost
any gas and even some liquids can be used as fuel, the most common
being hydrazine (N2H4). Hydrogen, nitrogen, ammonia and many other
fuels have also been used. For a given engine and power level, the
lighter the propellant the higher the specific impulse and the lower
the thrust. Hydrogen produces very high specific impulses (as high as
400 sec.) This may not sound very high but resistojets are designed
for small-thrust missions like orbital station keeping and the best
chemical engines in this range only have specific impulses of 200 sec
or less. They use so little power, 350 watts or less and then only
intermittently, that they can operate using residual electrical power
already available on the satellite. Because of their increased
specific impulse they need hundreds of pounds of fuel less than the
next best chemical engine. That's weight that can be used for more
propellant, so the satellite can remain on orbit years longer, or for
extra payload. Resistojets produce thrusts on the order of small
fractions of a pound.

a
space-qualified hydrazine resistojet

As
with any electrical device, resistojets are not perfectly efficient.
Typically they convert 50 percent of the electric energy passed
through them into thrust energy. (Don't sneer at this... the engine
in your car is at best only 25 percent efficient.)

Resistojets
can be scaled down to very small sizes, anywhere from ones that fit
in shoe boxes to others as small as a thimble, making it easy to
tailor them to any low thrust mission. Dozens of them are currently
in orbit helping satellites maintain their orbits. Next time you're
watching the Superbowl you can probably than a resistojet for keeping
the satellite transmitting the signal to be where it should be for
you to pick it up.

The
problem with resistojets is that the physical limitations of the
conductor means that the maximum temperature they can achieve is 1800
degrees C. Run them hotter than this and they start to melt. (Mission
directors hate it when that happens.) Fortunately, there's a
solution: the arcjet.

Arcjets:

An
arcjet is simply a resistojet where instead of passing the gas
through a heating coil it's passed through an electric arc.

diagram of an arcjet

Because
arcs can achieve temperatures of 15,00 degrees C. this means the
propellant gets heated to much higher temperatures (typically 3,000
degrees C.) than in resistojets and in so doing achieve higher
specific impulses, anywhere from 800 sec for ammonia to 2,000 seconds
for hydrogen. Arcjets tend to be higher power devices, typically 1 to
2 kilowatts, and used for higher thrust applications, like station
keeping of large satellites. Several are currently in orbit.

a
small space-qualified arcjet

hydrogen
arcjet firing

ammonium
arcjet firing

The
largest arcjet used in space was a 26 kilowatt engine operating on
ammonia with a specific impulse of 800 sec. It was part of the USAF's
ESEX space experiment program.

Arcjets
can run at up to 35 percent efficiency.

Two
problems hounds arcjets: the electrodes run glowing hot causing
erosion and this heat can get conducted to the spacecraft heating it
to unacceptable levels. For station keeping missions they aren't on
long enough for the heating to be a serious problem. But it could be
for large engines designed to operate for long periods of time.
Arcjets don't scale down as easily as resistojets and the smallest
are small shoe-box affairs.

Electrodeless
Electrothermal Engines:

There
are several variants of the resistojet or arcjet engine that use
microwaves or some sort of inductive coupling to heat the propellant.
They have some advantage in that power can be coupled directly to the
propellant without having to heat part of the engine. They suffer the
disadvantage that this power coupling may be inefficient and the the
microwave or inductive power itself may be inefficient to produce.
There are working models of these in laboratories but none have been
used to support and actual mission.

Electrostatic
Rocket
Engines:

Ion
engines:

Rub
a balloon against your hair or shirt and then hold it near your arm,
the hairs on your arm will feel tingly and be attracted to the
balloon. Bring the balloon near the carpet and bits of lint will be
pulled to it. What's happening is that electrons have been deposited
onto or removed from the balloon depending on what it was rubbed
against, giving it an electrostatic charge, which creates an
electrostatic field. A similar field can be used to produce thrust in
a rocket engine called an ion thruster.

ion engine diagram

As
propellant enters the ionization chamber (the small ns
on the left), electrons (small -sin
the middle)emitted
from the central hot cathode and attracted to the outer anode
collide with them knocking an electron off and causing the atoms of
the propellant to become ionized (+s on
the right). This means that they have an electric field around them
like the balloon. As these ions drift between two screens at the
right hand side of the ionization chamber, the strong electric field
of the "+" side repels them and the "-" side
attracts them, accelerating them to very high velocities. The ions
leave the engine and since the engine pushes on them to accelerate
them, they in turn push back against the engine creating thrust. Ion
thrusters typically use Xenon (A very heavy, inert gas) for
propellant, have specific impulses in the 3,000 to 6,000 range and
efficiencies up to 60 percent. An average thruster is one to two feet
in diameter, produces thrust on the order of small fractions of a
pound and weighs some tens of pounds.

a
typical ion engine

Downstream
of the exhaust is a hot cathode emitter that injects electrons into
the exhaust stream. Without this, the exiting ions would slowly cause
a charge to build up in the spacecraft that could interfere with its
operation and create a pull on the ions that would reduce the thrust.
You can see the small electron emitter in the upper right corner of
the picture above.

an
ion engine firing

Ion
thrusters are well developed and have been used on a few space
missions, such as a comet encounter. With their high specific
impulses they are well suited to deep space types of missions.

Colloid
Thruster:

A
colloid is a microdroplet like inkjet printers use to spray their
ink on paper. Given an electrical charge, these microdroplets, or
colloids, can be accelerated in a thruster similar to an ion
thruster. The advantage of a colloid thruster is that because the
individual particles being accelerated are so much larger and heavier
than the atoms in a regular ion engine, the specific impulse can be
lowered and thrust increased to make a better fit for a particular
mission. Also, the variety of propellants that can be used is much
greater. Although colloid thrusters have been around almost as long
as ion engines they have not been developed to flight status.
In the laboratory they typically have specific impulses around 1,000 sec.

diagram
of a single colloid thruster emitterMany
of these would be tied together to create a single large thruster.

Electromagnetic
Rocket Engines:

This
is by far the largest group of electric thrusters with many
different techniques used to create thrust. As widely divergent as
these thrusters may seem they all use the same principle: the Lorentz force.

If
you have an electric current flowing perpendicular to a magnetic
field, the magnetic field will push against the current. If the
current is flowing through a solid conductor or even a gas the gas
will be pushed out as well. This is the Lorentz force. Mount such a
device on a space ship and you have an electric rocket engine.

Rail
guns:

Drive
enough current down one side of two parallel conducting rails,
across a conductor that can slide down the rails while maintaining
contact, and up the other side and you have a rail gun. The current
flowing in the rails create a strong magnetic field between them. The
same current flowing across the sliding conductor is the current the
magnetic field wants to push away.

Pump
enough power into the thing and the sliding conductor will be
accelerated to thousands of feet per second. Mount the beast on a
tank and you you have an electric cannon. Put it on a spacecraft and
you have a rocket engine. You'll need to added something like a
machine gun feeder to supply it with a constant source of sliding
projectiles (which we should now call "propellant" since
we're using it as a rocket engine) but that's a simple mechanical
engineering problem.

a
weapon-type rail gun firing

While
such a propulsion system would work in principle, the length of the
rails makes a practical application of this concept difficult to
imagine. Also, as the sliding conductor accelerates down the rails
contact friction and erosion from arcing between the contacts erodes
the rails. For an engine such a device would have to fire repeatedly
over a long period of time and this erosion could be a problem.

Magnetoplasmadynamic
(MPD) Thrusters:

Crank
the power up on the rail gun high enough and the sliding conductor
would vaporize. The engine would still work because the plasma would
continue conducting current and be blown out the end of the gun.
Flatten and bend one of the rails around in a tube surrounding the
other rail and you have an MPD thruster. (For further details about
MPD thrusters please check out the MY
ELECTRIC ROCKET ENGINE
page on this site.)

MPD thruster diagram

MPD
thrusters are unique among the electric rocket engine fraternity
because they are capable of producing thrusts as high as 50 pounds in
an engine small enough to fix in a large shoe box. The problem with
them is the electrodes wear out from handling all the current and
they eat up enormous amounts of power: on the order of megawatts.
There is currently no space power system that comes even close to
this level. Typical performances numbers are 30 percent efficiency at
2,500 sec specific impulse. In laboratories they usually run on
argon, but anything that can be pumped into them can be used. (When I
was working in the lab I always wanted to run one of vaporized sodium
metal.) Using hydrogen would push the specific impulse into the
15,000 sec or higher range.

MPD
thruster firing

Because
of their compact size and potential for high thrust MPD thrusters
are one of the few viable options for primary propulsion on
high-mass, deep space missions. I like to think of them as being the
progenitors of the impulse engines of Star Trek fame.

Hall
Thrusters:

These
engines are popular with the Russians and over 100 have been used on
their space missions.

diagram of a Hall thruster

Electromagnets
around the outside cylinder and inside core create a magnetic field
pointing radially inward. The interplay of this magnetic field and
the electric field between the anode propellant injectors and the
electron cloud created outside of the thruster causes a current
(called the Hall current) to be induced to flow azimuthally around
the open annulus in the thruster. The magnetic field pushes on the
current and accelerates it, and the gas it's traveling through, out
of the thruster to create thrust.

Picture
of a Hall thruster.Note
the electron gun mounted on top of the thruster.

Typical
thrusters in US laboratories are a foot or so across, use 1 to 5
kilowatts of power, operate at 2,200 sec specific impulse, produce
less than one pound of thrust, and are 50 to 60 percent efficient.
They are noted for their durability.

Hall thruster firing

New designs
using vaporized bismuth can have efficiencies as high as 70 percent
making them the efficiency rulers in the electric propulsion world.

Hall thrusters
come in two main variants: the Stationary Plasma Thruster (SPT) and
the Thruster with Anode Layer (TAL). The SPT has insulating walls on
the acceleration chamber and is longer, the TAL has conducting
material lining the walls and is shorter.

Pulsed
Inductive Thrusters:

Imagine
an electromagnet sitting on its end on a table. Now place a metal
ring on top of it. Pulse current through the coil and a second
current will be induced to flow through the ring. This induced
current will flow around the ring in the direction opposite to the
coil. We now have another case of a current (flowing in the ring)
moving perpendicular to a magnetic field (created by the coil and
directed along the coil's axis) so the Lorentz law tells us there
will be a force on the ring wanting to push it away. If enough
current is forced through the coil the ring will shoot straight up
into the air. If the current in the coil is high enough and increases
fast enough, the ring will be vaporized and ionized. Even in the
gaseous state it'll still conduct the current and be accelerated away
from the coil. That's what a pulsed inductive thruster is.

Pulsed
Inductive Thruster

The
only difference between actual PIT thrusters and the coil analogy is
that a special valve and nozzle unit mounted in the center of the
coil directs a short pulse of gas down to cover the face of the coil.
The current pulse through the coil is synchronized with the gas pulse
so that the gas is ionized and accelerated away, creating thrust,
before it can dissipate into the vacuum of space.

Pulsed
inductive thrusters, are big, beautiful, sexy looking thrusters up
to a meter in diameter. They operate in a pulsed mode at up to 1,000
pulses per second with specific impulses between 2,000 and 5,000 sec
and thrusts of fractions of a pound to tens of pounds. Efficiencies
can be as high as 50 percent.

Although
the inductive coupling between the engine and plasma should imply
that there is no erosion as there is in the MPD thruster, at high
pulse rates the large surface area of the engine will be exposed to
the thermal loading of having a virtually constant plasma mere
centimeters from it. The coils could heat to the point where
increased electrical resistance could cause problems or even melting.
These engines are also power gluttons, eating up tens of kilowatts to
megawatts of electricity.

Pulsed
Plasma Thrusters:

These
small electric thrusters have been around for decades and have flown
on many space missions performing station keeping functions.

Pulsed
Plasma Thruster diagram

In
the pulsed plasma thruster, a bar of solid propellant (could be
anything but Teflon is the usual fuel of choice) is spring loaded
against two stops near the exit of the thruster. When it's desired to
fire it, a energy storage unit discharges an arc across the face of
the propellant, ablating a small amount of the Teflon bar. Just like
the rail gun and magnetoplasmadynamic thruster, the current flowing
through the vaporized propellant ionizes it and reacts with the
magnetic field created by the current to accelerate the propellant
out of the engine, creating thrust. As the propellant bar is eroded,
the spring pushes it forward for the next pulse.

Pulsed
Plasma Thruster

Pulsed
Plasma thruster firing

These
engines are extremely simple, reliable, and robust. They have to be
operated in the pulsed mode but can be pulsed rapidly to provide
almost continuous thrust. They typically use 30 watts or less power,
have efficiencies around 30 percent, specific impulses of 1,000
seconds, and thrust levels measured in micropounds to millipounds.

Field
Emission
Electric Propulsion (FEEP) Thrusters:

These
are extremely small thrusters that operate somewhat like a colloid
thruster in that they have a sharp propellant emitters. The
difference is that in the FEEP the emitter is so small that
individual ions are pulled from the emitter instead of droplets.
Also, ionization occurs as a side effect of the emission process so
an ionization chamber isn't required.

Field
Emission Electric Propulsion thruster

Because
the emitter hole or slit is so small, only 0.001 millimeters across,
capillary action both draws the liquid propellant into it and
prevents it from exhausting into space, therefore a valve is not required.

FEEPs
typically have specific impulses from 6,000 to 12,000 seconds and
use melted indium as a propellant.

Mass
Drivers:

A
mass driver is similar to a rail gun in that it accelerates a solid
projectile down a long runway. The difference is that a mass driver
uses pulsed electromagnets lined up down the length of the runway to
pull on the projectile, accelerating it to high speeds and thereby
generating thrust.

Mass
drivers are attractive because they can be extremely efficient (as
high as 95 percent) run cool and can be designed so there is no guide
rail wear. The problem is that they are so long, hundreds to
thousands of feet, and heavy that it's doubtful they could ever be
used for propulsion. But, if someone could develop a lightweight,
high temperature superconductor they may yet have a place in the
electric propulsion pantheon.

Electric
Propulsion Research Focal Points:

I
recommend that anyone wanting more detailed and up-to-date
information on electric propulsion contact someone at one or more of
the following organizations that are active in electric propulsion research:

University
of Michigan (http://www.engin.umich.edu/dept/aero/spacelab)

University
of Colorado

Princeton
University: the School of Mechanical and Aeronautical Engineering

Besides
the United States and Russia; Japan, Germany and Italy are active in
electric propulsion research.

In
Closing:

Hardly
a year goes by in the electric propulsion world without someone
thinking up a new concept. These are always variations of the
thrusters outlined in this page and attempt to get around one problem
or another through an innovative geometry, ionization scheme, or
other concept. It would be impossible to chronicle all of them but I
hope the thrusters that have been represented on this page provide a
basic understanding of the world of electric propulsion.

The
explanations of the thrusters on this page are oversimplifications
of what are in fact extremely complex devices. It takes a PhD and
many years of working with these devices to understand them at the
current state of the art.

The
bible for electric propulsion is the text: The Physics of
Electric Propulsion by Dr. Robert Jahn.

Electric
propulsion research is extremely expensive. While many of the
thrusters can be manufactured for a few thousands of dollars, the
enormous vacuum chamber required to test one of them can easily top
$1,000,000 to build and hundreds of thousands of dollars a year to operate.

(Return to the main
page
for more medical topics or to browse 80 other subjects: everything
from kaleidoscopes and metal detectors to the strange world of lucid dreaming.)